Recently, it has been feared that petroleum oil runs out. As a next stable energy-supply source used for a long time, thermal power generation using coal, which is rich in recoverable reserves and less in regional maldistribution, as fuel, has been expected to play an important role from now on.
On the other hand, minor elements (e.g., mercury (Hg) selenium (Se), arsenic (As), cadmium (Cd) and lead (Pd)) contained in an amount of 0.1% or less in exhaust gas, which is generated with combustion of coal, have known to be toxic to living organisms when they are present in excess of an allowable concentration. In particular, mercury, which is an element having a high vapor pressure, is easily emitted in the air. When exposed to a human body, even in a small amount, mercury causes neurological disorder, renal disorder and cardiovascular disorder. In addition, when inorganic mercury emitted in the air is absorbed by lakes and the sea, the inorganic mercury changes into organic mercury (methyl mercury). Such organic mercury, when it is absorbed into a human body via fishery products, produces teratogenic effects. For the reason, strict regulations are going to be imposed on discharge of mercury in countries all over the world centering on the United States.
Among mercury contained in the exhaust gas discharged from a thermal power generation plant, there are metal-form mercury and oxide-form mercury. Of them, the metal-form mercury is not fixed to smoke and other particles and thus remains in a gas phase at a temperature even close to the atmospheric temperature. Therefore, it is difficult to remove the metal-form mercury by an electric dust collector and a dusting machine such as a bag filter. On the other hand, the oxide-form mercury, in particular, mercury chloride, which is produced by reacting with hydrogen chloride or chlorine in exhaust gas, has a low vapor pressure and solubility and thus is easily fixed in ash and absorption liquid of desulfurization equipment. Therefore, the oxide-form mercury is relatively easily collected and removed by a dusting machine such as an electric dust collector and desulfurization equipment, etc.
In the circumstances, studies have been conducted for removing metal-form mercury by oxidizing it into oxide-form mercury having a low vapor pressure by use of a solid catalyst having a function of oxidizing mercury and then removing the oxide-form mercury by a dust collector and desulfurization equipment provided downstream. As such a solid catalyst, for example, common NOx removal catalysts (Ti—Mo—V, Ti—W—V), which contain titanium (Ti) as a main component and oxides of vanadium (V), molybdenum (Mo), tungsten (W) and the like as additional components, are known well. These catalysts are capable of efficiently oxidizing metal-form mercury into oxide-form mercury in the presence of a halogen compound (Patent Documents 1 and 2). Furthermore, Patent Document 3 discloses a method of removing nitrogen oxides by use of a catalyst whose catalytic activity is rarely lowered by the presence of volatile metal compounds in exhaust gas. Patent Document 4 discloses a catalyst for removing organic halogen compounds, which contains titanium oxide (TiO2), a titania-silica complex oxide (TiO2—SiO2) or a mixture of them and further contains a vanadium and molybdenum complex oxide.
In the Ti—Mo—V catalyst or Ti—W—V catalyst as mentioned above, the oxidation rate of metal mercury decreases when the temperature of exhaust gas is high and the concentration of a halogen compound in exhaust gas is low. To deal with the decrease of the oxidation rate, the amount of a catalyst may be increased. If the number of active centers is increased, the oxidation activity of metal mercury can be improved. However, this approach has a big problem in that oxidation activity of SO2 also augments with an increase of the amount of catalyst. Patent Document 3 and 4 are silent about removal of metal mercury by oxidation.
Patent Document 1: JP2005-125211A1
Patent Document 2: JP2003-53142A1
Patent Document 3: JP01-176431A1
Patent Document 4: JP2001-219066A1